[unreadable] [unreadable] Facioscapulohumeral muscular dystrophy (FSHD) is clinically characterized by progressive weakness of the facial, shoulder and upper arm muscles and is caused by contraction of the D4Z4 macrosatellite repeat in the subtelomere of chromosome 4q. In the US 15.000 people suffer from this disease for which there is no therapy. The pathogenic mechanism underlying FSHD is largely unknown but it is generally accepted that D4Z4 contraction causes a cascade of epigenetic events leading to transcriptional deregulation of one or more disease genes. [unreadable] One of the candidate genes for FSHD is FRG1. It is considered a good candidate for FSHD because of its localization close to D4Z4, its high conservation, and because muscle-specific overexpression of FRG1 in mice causes muscular dystrophy. Observations from our and other laboratories consistently suggest a role for FRG1 in RNA biogenesis, but its exact function is unknown. [unreadable] As FRG1 expression levels are high in mitotic active cells, FRG1 is upregulated in primary myoblast cultures of FSHD patients, high levels of FRG1 causes artificial nucleolar aggregates, and there is no faithful cell model for FSHD, we aim to study conditionally immortal myogenic cell lines with physiological relevant levels of FRG1-TAP overexpression, similar to what is observed in FSHD myoblasts. We propose to interrogate these cell models for the exact composition of the FRG1 ribonucleoprotein (RNP) complex at the protein and RNA level, during proliferation and in differentiation. We also propose to study the immediate effects of FSHD-like overexpression of FRG1 by quantitative (mRNA levels) and qualitative (exon use) transcriptome analysis, during proliferation and differentiation. [unreadable] We expect this study to yield detailed information about the FRG1 RNP complex in a myogenic context, during proliferation and differentiation and to yield information on the immediate effects of subtle FRG1 overexpression. This knowledge will be essential to better understand the pathogenic mechanism underlying FSHD. Moreover, on the long term, we expect that uniform myogenic cell models with well-defined disease parameters for FSHD will be of importance for pharmaceutical intervention studies. [unreadable] [unreadable] [unreadable]